1. Field of the Invention
The present invention relates in a general way to a process for obtaining a layer of single-crystal germanium on a substrate of single-crystal silicon.
2. Description of the Relevant Art
Silicon (Si) is the basic compound of microelectronics. It is currently available on the market in the form of wafers 200 mm in diameter. The performance limits of integrated circuits are in fact therefore those associated with the intrinsic properties of silicon. Among these properties, mention may be made of the electron mobility.
Germanium (Ge), which belongs to column IV of the Periodic Table of Elements, is a semiconductor. It is potentially more beneficial than Si since (i) it has a higher electron mobility, (ii) it absorbs well in the infrared range and (iii) its lattice parameter is greater than that of Si, thereby allowing heteroepitaxial structures using the semiconductor materials of columns III-V of the Periodic Table.
Unfortunately, germanium does not have a stable oxide and there are no large-diameter germanium wafers on the market, except at prohibitive prices.
Si1xe2x88x92xGex alloys have already been grown on substrates of single-crystal Si. The alloys obtained only rarely have germanium contents exceeding 50% in the alloy.
Moreover, when SiGe alloys are grown on single-crystal Si, the growth of the SiGe alloy is initially single-crystal growth. The greater the thickness of the layer and the higher its germanium content, the more the layer becomes xe2x80x9cstrainedxe2x80x9d. Above a certain thickness, the xe2x80x9cstrainxe2x80x9d becomes too high and the layer relaxes, emitting dislocations. These dislocations have a deleterious effect on the future circuits which will be constructed on this layer and the relaxation of the layers causes certain advantages of the strained band structure (offsetting of the conduction and valence bands depending on the strain states; Si/SiGe or SiGe/Si) to be lost. Corresponding to each composition and to each production temperature there is therefore a maximum thickness of strained layer.
In some applications, the concept of xe2x80x9crelaxed substratesxe2x80x9d has been developed, that is to say Si1xe2x88x92xGex layers are grown on silicon so as to exceed the critical thickness for a given composition, but by adjusting the deposition parameters for the layers so that the dislocations emitted do not propagate vertically but are bent over as to propagate in the plane of the layer in order subsequently to evaporate at the edges of the wafer. Growth therefore takes place from layers increasingly rich in germanium, it being possible for the germanium gradient to change stepwise or in a continuous fashion.
However, the films obtained by this xe2x80x9crelaxed substratexe2x80x9d process either have a relatively low ( less than 50%) degree of germanium enrichment or have an unacceptable emergent-dislocation density for applications in microelectronics.
Thus, the article entitled xe2x80x9cStepwise equilibrated graded GexSi1xe2x88x92x buffer with very low threading dislocation density on Si (001)xe2x80x9d, by G. Kissinger, T. Morgenstern, G. Morgenstern and H. Richter, Appl. Phy. Lett. 66(16), Apr. 13, 1995, describes a process in which the sequence of the following layers is deposited on a substrate:
250 nm Ge0.05Si0.95+100 nm Ge0.1Si0.9+100 nm Ge0.15Si0.85+150 nm Ge0.2Si0.8.
After deposition, each layer undergoes in situ annealing in hydrogen at 1095xc2x0 C. or 1050xc2x0 C. By way of comparison, similar sequences of layers have been deposited, but without annealing.
A 300 nm layer of GexSi1xe2x88x92x of the same composition as the upper buffer layer is also deposited on the latter.
The specimens that did not undergo intermediate annealing have an emergent-dislocation density of 106 cmxe2x88x922, whereas the specimen that did undergo annealing has an emergent-dislocation density of 103-104 cmxe2x88x922.
The article entitled xe2x80x9cLine, point and surface defect morphology of graded, relaxed GeSi alloys on Si substratesxe2x80x9d, by E. A. Fitzgerald and S. B. Samavedam, Thin Solid Films, 294, 1997, 3-10, describes the fabrication of relaxed substrates comprising up to 100% germanium. However, the process employed takes a long time (more than about 4 hours per wafer) and is consequently unattractive from an industrial standpoint. Moreover, this process is not reversible, that is to say it does not allow pure silicon to be deposited on a germanium substrate.
Furthermore, during the fabrication of such relaxed substrates, a surface roughness is observed which increases depending on the deposition conditions and which may have negative effectsxe2x80x94since they are cumulativexe2x80x94that is to say the onset of roughness can only become greater as the deposition proceeds.
A deposition process has also been proposed which makes it possible to form, on a silicon substrate, Si1xe2x88x92xGex layers (x varying from 0 to 1), possibly going as far as a layer of pure Ge, and having a low emergent-dislocation density.
The essential characteristic of this process includes, during the chemical vapor deposition, continuously modifying the stream of active gases (SiH4 and GeH4, for example) at the same time as the deposition temperature is varied. Thus, the emitted dislocations are rapidly rejected and removed in order to gradually relax the growing layer. Thus, it is possible to obtain these relaxed substrates going from a Ge concentration of zero (Si substrate) to a Ge concentration of 100% with a 4 to 5 xcexcm film whereas the prior techniques require interlayers of more than 10 xcexcm (typically about 25 xcexcm).
The advantages of the latter technique are therefore a smaller thickness of interlayer, so as to obtain a relaxed-substrate surface layer, and a low defect (emergent dislocation) density of about 105 defects/cm2 (compared with 106 for the prior processes).
However, this process still requires the deposition of an interlayer having a Ge concentration gradient, which requires film thicknesses of about 4 to 5 xcexcm.
Moreover, this technique also requires long deposition times, of more than one hour per wafer treated in certain cases, thereby reducing the hourly output of wafers and increasing the fabrication cost of the wafers.
Disclosed herein is a novel process for depositing a layer of pure single-crystal germanium on a substrate of single-crystal silicon, which does not require the deposition of an interlayer with a concentration gradient.
Disclosed herein is also such a deposition process giving low densities of residual emergent dislocations, of less than 103 defects/cm2 of surface.
Additionally such a process allows a layer to be obtained in a very short time and with small thickness (about 10 minutes for a 1 xcexcm layer of pure Ge).
According to a first method, the process for forming a layer of pure single-crystal germanium on a substrate of single-crystal silicon includes:
(a) temperature stabilization of the single-crystal silicon substrate at a first predetermined stabilized temperature (T1) of 400xc2x0 C. to 500xc2x0 C., preferably of 430xc2x0 C. to 460xc2x0 C.;
(b) chemical vapor deposition (CVD) of germanium at said first predetermined temperature (T1) until a germanium base layer with a predetermined thickness of less than a desired final thickness obtained on the substrate;
(c) increase in the germanium chemical vapor deposition temperature from the first predetermined temperature (T1) to a second predetermined temperature (T2) ranging from 750xc2x0 C. to 850xc2x0 C., preferably from 800xc2x0 C. to 850xc2x0 C.; and
(d) continuation of the germanium chemical vapor deposition at said second predetermined temperature (T2) until the desired final thickness of the single-crystal germanium layer is obtained.
According to a second method, the process for forming a layer of pure single-crystal germanium on a substrate of single-crystal silicon comprise, after step (c) and before step (d):
(c1) a step in which the germanium CVD deposition is stopped and the temperature is lowered from the second predetermined temperature (T2) down to a third predetermined temperature (T3) of 500 to 600xc2x0 C., preferably of 540 to 560xc2x0 C.:
(c2) a step of chemical vapor deposition at the third predetermined temperature (T3) of an Si1xe2x88x92xGex alloy, where xxe2x89xa60.9, until an Si1xe2x88x92xGex alloy interlayer having a predetermined thickness is obtained;
(c3) a transition step in which, at the third predetermined temperature (T3), the chemical vapor deposition of the Si1xe2x88x92xGex alloy changes over to chemical vapor deposition of pure Ge;
(c4) a step in which the CVD deposition of germanium is continued at said third predetermined temperature (T3) so as to obtain a multiplayer stack comprising the germanium base layer, and Si1xe2x88x92xGex alloy interlayer and a germanium top layer, the thickness of the stack being less than the desired final thickness; and
(c5) a step in which the germanium chemical vapor deposition temperature is increased from the third predetermined temperature (T3) up to a fourth predetermined temperature (T4) of 750 to 850xc2x0 C., preferably 800 to 850xc2x0 C.;
step (d) being carried out at this fourth predetermined temperature (T4), which is identical to or different from second predetermined temperature (T2), but preferably identical.
For chemical vapor deposition of pure germanium, any germanium precursor gas such as GeH4 may be used. As is well known, the germanium precursor gas is diluted with a carrier gas such as hydrogen. The dilution factors may vary from 10 to 1000. Preferably, the GeH4/H2 volume ratio is 10%.
Germanium deposition is preferably carried out at atmospheric pressure, since when the total pressure is less than 500 hPa the films deposited very quickly become rough and the emergent-dislocation density increases.
Step (a) of stabilizing the temperature of the silicon substrate is carried out in the absence of any reactive gas, but with the carrier gas, generally H2, present.
In all the chemical vapor deposition steps it is preferred to use, as carrier gas, H2 (purified or not) with a flow rate of about 20 1/minute.
In the pure germanium deposition steps, the precursor gas is preferably GeH4 and the flow rate is generally between 30 and 400 cm3/minute under standard conditions, the optimum value being 300 cm3/minute (of course, these are values of the nominal flow rate of GeH4 diluted to 10% by volume in H2).
The durations of the germanium CVD deposition steps are, of course, determined according to the desired thickness of the final germanium layer.
Thus, in the first method, under the above preferred conditions and with a duration of 10 minutes for step (b), of 60 seconds for step (c) and of 120 seconds for step (d), a final layer of about 1 xcexcm of pure single-crystal germanium having an extremely low emergent-dislocation density, possibly of less than 10 defects/cm2, is obtained.
In the case of the second method, all the steps are also carried out in the presence of a carrier gas, preferably hydrogen, and also preferably at atmospheric pressure.
Step (c1) of lowering the temperature is carried in the absence of reactive precursor gases but in the presence of carrier gas, for example hydrogen.
Apart from the deposition temperatures, germanium CVD deposition is carried out under the same conditions as above.
The CVD deposition of the Si1xe2x88x92xGex alloy layer is carried out using a mixture of germanium precursor and silicon precursor gases in the desired proportions for obtaining a film of Si1xe2x88x92xGex alloy containing at least 90 wt. % germanium. The recommended germanium precursor gas is GeH4. The recommended silicon precursor gases are SiH4, Si2H6, SiH2Cl2, SiHCl3, SiCl4 and Si (CH3)4, SiH4 being preferred.
The SiGe alloy interlayer will generally have a thickness of between 5 and 10 nm, preferably about 10 nm, and of course the CVD conditions for depositing this layer will be chosen so as to meet the requirements in terms of thickness and germanium content of the layer. In particular, if the germanium content of this SiGe alloy interlayer is less than 90 wt%, the emergent-dislocation density increases.
The process may include, prior to step (a) of stabilizing the temperature of the substrate, a step of impregnating the surface of the substrate by CVD vapor phase deposition of a silicon layer at a temperature of 500 to 600xc2x0 C., preferably 550xc2x0 C. This CVD deposition step is also preferably carried out at atmospheric pressure. The preferred precursor gas is SiH4 and, as is well known, the deposition is carried out in the presence of a carrier gas, preferably hydrogen.
The thickness of the silicon impregnation layer is generally from 1 to 5 nm, preferably about 3 nm.
Although this is not very well understood, the presence of such an impregnation layer reduces the emergent-dislocation density in the final product.
Also preferably, the surface of the substrate is subjected to a preparation step prior to implementing the process.
This preparation step may conventionally be a surface cleaning step, for example any process in a liquid or gas phase which cleans the silicon surface of metallic and organic residues, such as the conventional SCl (NH4OH+H2O2) and SC2 (HCl+H2O2) solutions, or even H2SO4+H2O2. In all cases, the cleaning is completed by a treatment phase using a dilute aqueous HF solution followed by rinsing with water.
The products obtained by the process generally have an emergent-dislocation densityxe2x89xa6103/cm2 and possibly even less than 10 defects/cm2.
Although the process described above limits the appearance of surface roughness, it is still desirable to reduce the surface roughness of the germanium film.
To eliminate the roughness, it is possible to use any type of chemical-mechanical polishing conventionally used in silicon technology.
The principle of chemical-mechanical polishing is known and conventional and includes rubbing the wafer to be polished on a cloth imbibed with an abrasive, applying pressure and moving this wafer with respect to the cloth. The combined mechanical and chemical effects remove molecules of the polished material preferentially in the raised areas and planarizes the material to be polished.
The polishing is controlled either in situ, by controlling the polishing data such as the motor current, or ex situ qualitatively by optical or microscopical observation and/or quantitatively by using the technique of atomic force microscopy [measurement of the (rms) or peak/valley average roughness].
After polishing, mechanical residues (encrusted matter) may remain on the surface and these are removed by mechanical brushing and rinsing.
After this cleaning, the polishing may leave a disturbed area on the surface and a treatment to regenerate the surface may be necessary. This treatment, which is of the etching type, must nevertheless be carried out without causing the entire active layer to disappear. Several procedures are possible.
It is possible either (i) for the layer to undergo dry or wet etching or (ii) for the surface to be oxidized and then for the oxide to be dissolved. These two means use the extreme sensitivity of Ge to oxygen (oxygen gas or ozone or ozone dissolved in water, or oxygen plasma, etc), Ge oxides being volatile or unstable.
After these treatments, it is possible to resume epitaxy on the surface, particularly according to the process of the invention described above. In this preferred case, the desired surface finish (number of defects), and hence a xe2x80x9cguaranteedxe2x80x9d layer, is obtained immediately, it being possible for the thickness of the layer to be adjusted according to the application envisioned. In addition, the impurities are trapped by the subjacent dislocation network.
The processes described above therefore make it possible to obtain germanium layers of single-crystal structure directly deposited on silicon which are of very high quality and which allow further deposition by heteroepitaxy of elements of Groups III to V of the Periodic Table and of compounds of these elements (III-V heteroepitaxy) such as GaAs.
However, the Ge layers obtained may be under a slight strain (lattice parameter slightly less than that of bulk Ge) detrimental to subsequent growth of a heteroepilayer, for example of GaAs on Ge. The Ge layer could relax this strain during a subsequent temperature rise, but this would have the regrettable effect of making the surface rough again and therefore of impeding the subsequent III-V heteroepitaxial growth, for example by creating defects.
In order to avoid this latter effect, the particularly preferred processes include a step of stabilizing the germanium layer. This stabilization step, introduced at the end of growth of the germanium layer (before the chemical-mechanical polishing), has the effect (1) of relaxing the strains and recovering the theoretical lattice parameter of germanium and (2) of consequently stabilizing the structure during subsequent annealing treatments.
More specifically, this stabilization step consists of an annealing treatment in a hydrogen atmosphere at a temperature ranging from 650xc2x0 C. to less than 900xc2x0 C. for a sufficient time, generally about minutes or longer, in order to eliminate the residual strain. The duration of the annealing treatment obviously depends on the annealing temperature and on the thickness of the germanium layer.
It is preferable that the annealing temperature be below 900xc2x0 C., since above 900xc2x0 C. germanium, which melts at 937xc2x0 C., becomes very unstable.
This stabilization step may be carried out in a conventional multiwafer furnace; however, it is preferably carried out in situ (after growing the germanium layer) so as to avoid any contaminations by carbon and oxygen species in a single-wafer reactor.
After annealing, the germanium layer may undergo a chemical-mechanical polishing treatment as described above.
After cleaning the wafers, for example using a bath of oxonated water, a bath of dilute HF and drying under IPA, a heteroepilayer, for example of Ge or GaAs, may be grown.